This is the sort of event I can never, ever manage to predict. Like the Harvard OA mandate. Or the PRISM Coalition.
In brief, Nature Publishing Group tried the usual big-publisher contract-renewal tactics: jack the price a lot, because although librarians squeal, faculty never listen, so eventually the librarians knuckle under and sign the big fat check.
Only this time? Not only is check signage at risk, but so is all the free labor that University of California faculty provide to NPG in the form of authoring, editing, and peer review. That latter is the real boycott, and everyone involved knows it.
Pass me the popcorn. This is getting good.
Edited to add: Very nice writeup by Jen Howard in the Chronk. You must read all the way through to the end; that final quote is a lulu.